This invention relates to a method of reacting chemicals. In one embodiment it relates to a method of driving chemical synthesis of small molecules such as chemical intermediates, but can be used to drive destruction of others such as pollutants.
Hydroxyl radicals HO* are useful in the polymerisation of a variety of commercial polymers, oxidative synthesis of various organic compounds and destruction of water pollutants [1-3]. In some of these reactions such as polymerisation via a chain reaction mechanism, hydroxyl radicals act as an initiator and the amount required is fairly small. In other reactions, such as the synthesis of phenol via direct hydroxylation of benzene, hydroxyl radicals are consumed as a reagent in large quantities. There are also cases in which HO* consumption is not only in large quantities but also at high concentrations. An important example is the dimerization of pivalic acid in the synthesis of tetramethyladipic acid [4,5]. The key reaction step is the dimerization of two radicals of the substrate produced through hydrogen atom abstraction by HO*. In order for such a bimolecular radical-radical reaction to proceed at a rate which is sufficient for synthetic purposes, HO* must be present at a relatively high concentration. This presents difficulties owing to the relatively slow kinetics of the chemical reactions utilised. Hydroxyl radicals react readily with a variety of compounds often at a rate close to diffusion control (the rate constants of hydrogen abstraction and electrophilic substitution reactions, for example, are in the range 108-1010 Mxe2x88x921sxe2x88x921) [6-8]. If the reaction conditions are too intense, the radicals generated tend to react indiscriminately and the reaction becomes uncontrollable. Therefore, there is a need for developing a catalytic scheme in which radicals such as HO* can be generated efficiently under relatively mild conditions so as to allow better control of the reaction.
The invention provides a method of reacting chemicals, the method comprising:
exposing at least one metal electrode to one or more different energies from the group comprising:
ultrasonic energy, electrochemical energy, electrical energy, thermal energy, and visible and invisible radiation;
generating hydroxyl radicals from any suitable source; and
reacting the hydroxyl radicals with one or more reactants.
Preferably the metal electrode is exposed to electrochemical energy.
Preferably the metal electrode is also exposed to ultrasound energy. Preferably a cup-horn apparatus is used to apply the ultrasound energy.
In one embodiment the present invention provides a method of reacting chemicals, the method comprising:
exposing the metal electrodes to electrochemical energy;
generating hydroxyl radicals; and
reacting the hydroxyl radicals with one or more reactants.
Exposure to the electrochemical and ultrasound energies is preferably conducted under controlled conditions.
Preferably there are two electrodes. Preferably the electrodes comprise iron but may comprise any one or a combination of two or more metals chosen from iron, cobalt, copper, cerium, nickel, stainless steel, manganese, vanadium, molybdenum, titanium, platinum, iridium, osmium, ruthenium, gold and silver.
Optionally a first electrode comprises Fe2+/Fe3+ in conjunction with Ce3+/Ce4+ and a second electrode comprises a noble metal electrode, and the following reaction takes place at the first electrode:
Fe3++Ce3+xe2x86x92Fe2++Ce4+
This process can form part of the recycling of iron ions when Fe2+ is consumed in a Fenton-like reaction forming Fe3+ and HOxe2x88x92.
The method may be used to decompose chemicals such as in the destruction of pollutants etc or alternatively the method may be used in the synthesis of chemicals.
In a preferred embodiment the present invention provides a method of chemically synthesising a reaction product from reactants, said method comprising the following steps:
a) electrochemically forming Fe2+ ions;
b) exposing said Fe2+ ions to H2O2 to produce HO*;
c) exposing said HO* to the reactants in conditions suitable for said HO* and said reactants to chemically react together to promote formation of said reaction product.
Preferably the reaction mixture of any of steps a), b) and/or c) is also exposed to ultrasonic radiation.
The reactants are preferably heated, and can be exposed to ultrasonic and electrochemical energy at the same time or consecutively, optionally at an given temperature. Where the reactants are exposed to ultrasound and electrochemical energy consecutively they are preferably first exposed to electrochemical energy.
Fe2+ ions are preferably generated through electrochemical dissolution of an iron anode in electrolytes of eg acetonitrile-water mixtures. In this way, the rate of Fe2+ generation can be controlled conveniently by adjusting the electrode potential, current, or the composition of the electrolyte. The pristine ion Fe2+ produced reacts instantly with the substrate (eg H2O2) before reaching salvation equilibrium and therefore is more potent than that delivered traditionally via the addition of a ferrous salt solution prepared in advance, in which the iron (II) ions are completely solvated in water, aqueous acid or acetonitrile-aqueous acid mixtures.
The method preferably involves generating hydroxyl radicals in a Fenton-like process (Fe2++H2O2xe2x86x92HO*+HOxe2x88x92+Fe3+) [9]. The generated radical can be utilised in a hydroxylation reaction, and typically in an oxidative hydroxylation reaction.
The Fenton process is attractive in that the reagents used are inexpensive, and no special facilities such as an electron accelerator (used in pulse radiolysis) are required [9].
Alternatively the method may comprise generating the hydroxyl radicals by reacting the metal with hydrogen peroxide by the following equation:
M+H2O2xe2x86x92HO*+HOxe2x88x92+Mn++(nxe2x88x921)exe2x88x92
wherein M is Fe or another metal and n is any value from 1 to 7.
It is important that the reaction produces a hydroxyl radical as part of the process, but otherwise parameters can be varied.
The invention preferably involves the use of catalytic schemes [10], electrochemical regeneration [11] of Mn+, (for example Fe+2Fe3+xe2x86x923Fe2+) with or without the synergistic coupling of ultrasonic energy and the optional use of an unreactive solvent such as acetonitrile [10] in order to ameliorate the need for constant addition and consumption (rather than recycling) of ferrous salts, limited control of the reaction due to simple use of chemical and thermal energies alone and the need for an aqueous medium in which the solubility of most organic substrates is often too low.
The free hydroxyl radical preferably reacts with a carbon based reactant to remove a hydrogen atom and generate a carbon-based radical as a result. The carbon-based radical can then dimerize or react with other carbon-based radicals or other reactants.
The method is preferably used in synthesis, but can also be useful in the destruction of pollutants etc. In certain embodiments of this nature, the use of external current applied to the system is not necessary, and a simple galvanic cell can be set up.